Microcontrollers control and monitor the overall operation of the air-conditioning system. By using a serial communication circuit to relay vital information to one another, microcontrollers are able to make the necessary decisions concerning the process being controlled. When a glitch occurs in the system, the communication signals exchange between the indoor unit and outdoor unit can be intercepted for diagnosis. By means of a data logger, the intercepted data can be recorded and viewed at a later time. Computer based data logging systems can be used for this purpose. It is common for a serial port or RS232 port of the computer to be used for receiving data. For the purpose of relaying the data transmitted by the air-conditioning system to the computer, an RS 232 converter circuit is required.
Generally, an RS232 transceiver integrated circuit such as the MAX232 and an opto-coupler can be used to perform the RS232 conversions as shown in FIG. 1. The opto-coupler first converts the logical pulses of current of the signal line to Transistor-Transistor Logic (TL) signals. The RS 232 transceiver integrated Circuit (IC) in turn, converts the TL voltage levels to RS 232 level voltages, which are then fed into the serial port of the computer. The transceiver IC uses charge pump circuits to step up the voltage in order to generate the RS 232 level signals from the TTL signals. These charge pump circuits however, require external capacitors to multiply the voltage. Usually, four capacitors with values between 0.1 pF to 1 pF need to be added externally to the transceiver IC in order for it to operate properly. The use of external capacitors will take up space on the printed circuit board. In addition to that, the transceiver IC also requires a stable external power supply typically in the region +5V for operation.
The Electronic Industries Alliance (EIA) standard RS 232 requires a receiver to distinguish between voltages as low as +3V and −3V, where positive level voltages are defined as logic ‘0’ whereas negative level voltages are defined as logic ‘1’. Typically, the positive level voltages can be in the range from +5V to +12V while the negative level voltage ranges from −5V to −12V, depending on the hardware. The large voltage swing between positive and negative is intended to make the transmission of data signals less susceptible to electrical noise.
It is an object of the circuits described herein to convert the data signal line from the air-conditioner, which is in the form of logical pulses of current to the RS232 voltage signals on the personal computer.
It is another object of the circuits described herein to troubleshoot the communication signals between the indoor unit and outdoor unit of an air-conditioner via the conversion circuit.